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BIOMOLECULES

Chapter Overview

Biomolecules are the chemical compounds found in living organisms. This chapter covers the chemical analysis of cells, types of biomolecules (carbohydrates, proteins, lipids, nucleic acids), their structure, and the detailed study of enzymes. This is one of the most high-yield chapters for NEET UG.

Reference: NCERT Class 11 Biology (kebo109.pdf) | Lehninger — Principles of Biochemistry | B.R. Vashist — Biochemistry

1. Chemical Analysis of Cells

To determine what chemicals are present in living tissue, a chemical analysis procedure is used:

Analytical Method

Take living tissue → grind in trichloroacetic acid (TCA) → get a slurry
Filter → Filtrate = Acid-soluble fraction (small molecules: amino acids, glucose, water, ions, nucleotides)
Retentate = Acid-insoluble fraction (macromolecules: proteins, nucleic acids, polysaccharides, lipids)

The biomolecules in the acid-insoluble fraction are called Biomacromolecules — they have molecular weights in thousands to millions of Daltons.

📷 Fig. 9.1 (NCERT) Bar graph showing distribution of compounds in acid-soluble (pool) and acid-insoluble (macromolecular) fractions of a cell NCERT Figure 9.1

1.1 Primary vs Secondary Metabolites

Feature	Primary Metabolites	Secondary Metabolites
Definition	Compounds involved directly in normal growth, development, and reproduction	Compounds NOT directly involved in primary metabolic processes
Role	Essential for life	Ecological functions — defence, attraction, competition
Examples	Amino acids, nucleotides, sugars, vitamins, fatty acids, ethanol	Rubber, gums, resins, alkaloids, terpenes, essential oils, pigments, toxins
Found in	All organisms	Mostly plants and microbes; few in animals
Commercial use	Food, pharmaceutical basics	Drugs (morphine, quinine), dyes (indigo), spices, fragrances

2. Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones or compounds that produce them on hydrolysis. General formula: (CH₂O)ₙ. They are the most abundant biomolecules on Earth.

2.1 Classification of Carbohydrates

A. Monosaccharides (Simple Sugars)

Monosaccharides

Cannot be hydrolysed further into simpler sugars
Classified by: number of carbons (triose C₃, tetrose C₄, pentose C₅, hexose C₆) and functional group (aldose = aldehyde; ketose = ketone)
Key examples:
- Glucose (C₆H₁₂O₆) — Aldohexose; most important energy source; occurs as D and L forms
- Fructose (C₆H₁₂O₆) — Ketohexose; isomer of glucose; found in fruits, honey
- Galactose (C₆H₁₂O₆) — Aldohexose; component of lactose
- Ribose (C₅H₁₀O₅) — Aldopentose; component of RNA
- Deoxyribose (C₅H₁₀O₄) — Aldopentose; component of DNA (one oxygen less than ribose)
Reducing sugars: Have free aldehyde/ketone group → reduce Benedict's or Fehling's solution (e.g., glucose, fructose, galactose, maltose, lactose)
Non-reducing sugars: No free aldehyde/ketone (e.g., Sucrose — glycosidic bond between C1 of glucose and C2 of fructose blocks both anomeric carbons)

B. Oligosaccharides

2–10 monosaccharide units joined by glycosidic bonds (condensation/dehydration)
Disaccharides (2 units):
- Sucrose = Glucose + Fructose (α1→β2 glycosidic bond) — table sugar; non-reducing
- Maltose = Glucose + Glucose (α1→4 bond) — malt sugar; reducing
- Lactose = Galactose + Glucose (β1→4 bond) — milk sugar; reducing
- Cellobiose = Glucose + Glucose (β1→4 bond) — unit of cellulose
- Trehalose = Glucose + Glucose — insect blood sugar
Trisaccharide: Raffinose (Galactose + Glucose + Fructose) — in sugar beet

C. Polysaccharides (Biomacromolecules)

Important Polysaccharides

Polysaccharide	Monomer	Bond	Function	Found In
Starch	α-D-Glucose	α1→4 (amylose); α1→4 + α1→6 (amylopectin)	Storage carbohydrate	Plants (potato, rice, wheat)
Glycogen	α-D-Glucose	α1→4 + α1→6 (highly branched)	Storage carbohydrate (animals)	Liver, muscle cells
Cellulose	β-D-Glucose	β1→4	Structural (cell wall)	Plant cell walls
Chitin	N-acetylglucosamine (NAG)	β1→4	Structural (exoskeleton, cell wall)	Fungi, insect exoskeleton, crustaceans
Inulin	Fructose	β2→1	Storage	Dahlia tubers, chicory
Agar-agar	Galactose derivatives	—	Culture medium, food	Red algae (Gelidium)
Heparin	GlcUA + GlcNAc	—	Anticoagulant	Mast cells, liver
Hyaluronic acid	GlcUA + GlcNAc	—	Synovial fluid, connective tissue	Animal connective tissue

Amylose vs Amylopectin (in Starch): Amylose — unbranched, α1→4 links only, 15–20% of starch. Amylopectin — branched (branch at every 24–30 glucose via α1→6 links), 80–85% of starch.

Starch gives blue-black colour with iodine (iodine fits into helical amylose) — used as test for starch.

🎯 NEET PYQ Focus

NEET 2023

Which polysaccharide is made of β-D-glucose monomers with β1→4 linkage?
Answer: Cellulose (structural carbohydrate of plant cell wall)

NEET 2022

Glycogen is called "Animal starch" because:
Answer: Both are made of α-glucose, but glycogen is more branched (branch every ~8-10 units vs every 24-30 in amylopectin)

NEET 2020

Sucrose is a non-reducing sugar because:
Answer: Both anomeric carbons (C1 of glucose and C2 of fructose) are involved in glycosidic bond — no free aldehyde/ketone group

NEET 2019

Inulin is a polymer of:
Answer: Fructose

3. Proteins

Proteins are the most abundant organic molecules in living cells (~50% of dry weight). They are polymers of amino acids linked by peptide bonds. They are the most diverse class of biomolecules in structure and function.

3.1 Amino Acids — Building Blocks of Proteins

Structure of Amino Acids

Each amino acid has: Central carbon (α-C) bonded to:
- Amino group (—NH₂)
- Carboxyl group (—COOH)
- Hydrogen atom (—H)
- R group (side chain) — variable; determines the type and properties of amino acid
There are 20 standard amino acids that make up proteins
Essential amino acids (cannot be synthesised in the body; must be taken in diet): PVTTIMHLL — F → Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, Lysine + sometimes Arginine
Non-essential amino acids: Can be synthesised by the body (e.g., Glycine, Alanine, Glutamine, Serine)

📷 Fig. 9.2 (NCERT) General structure of an amino acid — showing central alpha-carbon, amino group (—NH₂), carboxyl group (—COOH), hydrogen, and R (variable side chain) NCERT Figure 9.2

Classification of Amino Acids	Property of R group	Examples
Non-polar / Hydrophobic	Aliphatic or aromatic, no charge	Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Phenylalanine, Tryptophan, Methionine
Polar / Uncharged	Polar but no net charge at physiological pH	Serine, Threonine, Cysteine, Asparagine, Glutamine, Tyrosine
Positively charged (Basic)	Positive charge at pH 7.0	Lysine, Arginine, Histidine
Negatively charged (Acidic)	Negative charge at pH 7.0	Aspartate (Aspartic acid), Glutamate (Glutamic acid)
Special	Unique structural properties	Glycine (smallest, no chiral centre), Proline (cyclic — imino acid), Cysteine (can form S–S bonds)

3.2 Peptide Bond

Peptide Bond Formation

A peptide bond is a covalent bond formed between the carboxyl group (—COOH) of one amino acid and the amino group (—NH₂) of another amino acid with the release of one water molecule (condensation reaction).

A chain of amino acids linked by peptide bonds = polypeptide
A protein has a free N-terminus (H₂N—) and a free C-terminus (—COOH)
Peptide bond has partial double bond character (resonance) — planar, rigid, and resistant to rotation
Protein sequence is always written N → C terminus

📷 Fig. 9.3 (NCERT) Formation of a dipeptide by condensation of two amino acids — showing peptide bond (—CO—NH—) and release of water molecule NCERT Figure 9.3

3.3 Four Levels of Protein Structure

📷 Fig. 9.4 (NCERT) Four levels of protein structure — Primary (linear sequence), Secondary (α-helix and β-sheet), Tertiary (3D folding), Quaternary (multiple subunits) NCERT Figure 9.4

Level	Description	Bonds Involved	Example
Primary	Linear sequence of amino acids (N→C terminus)	Peptide bonds only	Any polypeptide chain; sickle cell haemoglobin (Glu→Val mutation)
Secondary	Regular, repeating local structures formed by backbone H-bonding	Hydrogen bonds (between —C=O and —N—H of backbone)	α-helix (right-handed, 3.6 residues/turn); β-pleated sheet (parallel or anti-parallel)
Tertiary	Overall 3D folding of a single polypeptide chain	H-bonds, Disulfide bonds (S–S), Hydrophobic interactions, Van der Waals, Ionic/electrostatic bonds	Myoglobin, Lysozyme, Ribonuclease
Quaternary	Arrangement of multiple polypeptide subunits (protomers)	Same as tertiary (non-covalent interactions between subunits)	Haemoglobin (4 subunits: 2α + 2β), Collagen (triple helix)

3.4 Classification of Proteins

Basis	Types	Examples
Shape	Fibrous (elongated, insoluble) vs Globular (compact, spherical, soluble)	Fibrous: Collagen, Keratin, Actin, Myosin \| Globular: Haemoglobin, Enzymes, Hormones (insulin)
Function	Structural, Enzymatic, Transport, Hormonal, Antibodies, Contractile, Regulatory, Storage	Keratin (structural), Pepsin (enzymatic), Haemoglobin (transport), Insulin (hormonal)
Composition	Simple (only amino acids) vs Conjugated (protein + non-protein prosthetic group)	Simple: Albumin \| Conjugated: Haemoglobin (haem group = prosthetic group), Glycoprotein, Lipoprotein

🎯 NEET PYQ Focus

NEET 2023

Secondary structure of protein is stabilised by:
Answer: Hydrogen bonds (between C=O and N-H groups of the polypeptide backbone)

NEET 2022

Which protein has quaternary structure?
Answer: Haemoglobin (2α + 2β subunits)

NEET 2021

What type of bond links amino acids in a protein?
Answer: Peptide bond (covalent bond between —COOH and —NH₂ with loss of H₂O)

NEET 2020

Proline is unique among amino acids because:
Answer: It is an imino acid (—NH— instead of —NH₂); its side chain forms a ring with the backbone nitrogen — disrupts α-helix

4. Lipids

Lipids are NOT true macromolecules (most are not polymers). They are insoluble in water but soluble in organic solvents (chloroform, ether, benzene). They are grouped together due to their hydrophobic nature.

4.1 Classification of Lipids

Types of Lipids

A. Fatty Acids

Long chain hydrocarbons with a terminal carboxyl group (—COOH)
Saturated fatty acids: No double bonds; maximum H atoms (solid at room temp) — e.g., Palmitic acid (C16), Stearic acid (C18)
Unsaturated fatty acids: One or more double bonds (C=C) — liquid at room temp (oils) — e.g., Oleic acid (C18:1), Linoleic acid (C18:2 — essential), Linolenic acid (C18:3 — essential)
Essential fatty acids: Linoleic acid and Linolenic acid — cannot be synthesised in body

B. Triglycerides (Fats and Oils)

Glycerol + 3 Fatty Acids linked by ester bonds
Fats = solid at room temp (saturated fatty acids) | Oils = liquid at room temp (unsaturated)
Function: Energy storage (9 kcal/g — highest energy density), insulation, protection

C. Phospholipids

Glycerol + 2 Fatty acids + Phosphate + Alcohol (choline, serine, ethanolamine)
Amphipathic — hydrophilic head (phosphate group) + hydrophobic tail (fatty acid chains)
Form bilayers — basis of all biological membranes
Examples: Lecithin (phosphatidylcholine), Cephalin (phosphatidylethanolamine)

D. Steroids (Sterols)

Based on steroid nucleus (four fused carbon rings)
Cholesterol — in animal cell membranes; precursor for bile acids, steroid hormones (cortisol, testosterone, oestrogen, aldosterone), Vitamin D
Plant sterols: Sitosterol, Stigmasterol

E. Waxes

Long chain fatty acid + long chain alcohol (ester bond)
Function: Waterproofing (cuticle of plants, beeswax, earwax)

⚡ Memory Trick — Lipids

"Lipids are SOS — Soluble in Organic Solvents" (not water)

"Phospholipids are AMP — AMPhipathic" — one end loves water, other end hates it → perfect for membranes

Energy order: Fats (9 kcal/g) > Proteins (4 kcal/g) = Carbohydrates (4 kcal/g)

5. Nucleic Acids

Nucleic acids are polymers of nucleotides (polynucleotides). They store and transmit genetic information and direct protein synthesis. Two types: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid).

5.1 Nucleotide Structure

Components of a Nucleotide

Each nucleotide = Phosphate group + Sugar + Nitrogenous base

Sugar: Ribose (in RNA) or Deoxyribose (in DNA) — pentose sugar
Nitrogenous bases:
- Purines (double ring): Adenine (A), Guanine (G) — present in both DNA and RNA
- Pyrimidines (single ring): Cytosine (C), Thymine (T) — only in DNA; Uracil (U) — only in RNA (replaces Thymine)
Nucleoside = Sugar + Base (no phosphate)
Nucleotide = Sugar + Base + Phosphate
Nucleotides are linked by 3'→5' phosphodiester bonds

📷 Fig. 9.5 (NCERT) Structure of a nucleotide — showing pentose sugar (ribose/deoxyribose), nitrogenous base, and phosphate group; and phosphodiester bond NCERT Figure 9.5

5.2 DNA vs RNA

Feature	DNA	RNA
Sugar	Deoxyribose (2'-deoxyribose)	Ribose
Bases	A, G, C, T	A, G, C, U (Uracil instead of Thymine)
Strands	Double-stranded (usually)	Single-stranded (usually)
Base pairing	A=T (2 H-bonds); G≡C (3 H-bonds)	A=U (2 H-bonds); G≡C (3 H-bonds)
Location	Nucleus, mitochondria, chloroplasts	Nucleus, cytoplasm, ribosomes
Function	Stores genetic information; template for replication & transcription	Translation of genetic info into proteins
Stability	More stable (no 2'-OH group)	Less stable (2'-OH makes it susceptible to hydrolysis)
Types	ds-DNA (main), some viruses have ss-DNA	mRNA, tRNA, rRNA, snRNA, miRNA, siRNA

5.3 Structure of DNA — Watson and Crick Model (1953)

DNA Double Helix

Proposed by James Watson and Francis Crick (1953), based on X-ray crystallography data of Rosalind Franklin and Maurice Wilkins.

Two polynucleotide strands coiled around each other in a right-handed double helix
Strands are antiparallel — one runs 5'→3', the other 3'→5'
Sugar-phosphate backbone on the outside; bases on the inside
Chargaff's rules: A = T and G = C (A + G = C + T, i.e., purines = pyrimidines)
Base pairing: A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds)
Helical parameters: Pitch = 3.4 nm per complete turn; base pairs per turn = 10; distance between adjacent base pairs = 0.34 nm
Diameter of helix = 2 nm
Major groove and minor groove visible in B-form DNA
Forms: B-DNA (physiological, right-handed); A-DNA (dehydrated, right-handed); Z-DNA (left-handed)

📷 Fig. 9.6 (NCERT) Watson and Crick DNA double helix model — showing antiparallel strands, sugar-phosphate backbone, A-T and G-C base pairs with hydrogen bonds, major and minor grooves NCERT Figure 9.6

🎯 NEET PYQ Focus

NEET 2023

In DNA double helix, A-T pair has how many H-bonds?
Answer: 2 hydrogen bonds (G-C has 3)

NEET 2022

Which base is present in RNA but NOT in DNA?
Answer: Uracil (U) — replaces Thymine in RNA

NEET 2021

The number of base pairs per turn in B-form DNA is:
Answer: 10 base pairs per turn; pitch = 3.4 nm

NEET 2019

Which of the following is a purine?
Answer: Adenine and Guanine — double ring structure. Pyrimidines (single ring): Cytosine, Thymine, Uracil

6. Enzymes — Detailed Study

Enzymes are biological catalysts — they speed up biochemical reactions without being consumed. Almost all enzymes are proteins (exceptions: Ribozymes — catalytic RNA molecules). Enzymes lower the activation energy of reactions.

Key Enzyme Terminology

Substrate (S): The molecule on which an enzyme acts
Active site: The specific region of the enzyme where the substrate binds; has two regions: binding site (for substrate) and catalytic site (where reaction occurs)
Enzyme-Substrate Complex (ES): Temporary complex formed when substrate binds to active site
Product (P): The result of the enzyme-catalysed reaction
Activation energy: The minimum energy required to initiate a reaction — enzymes lower this barrier
Cofactors: Non-protein components required by some enzymes for activity:
- Prosthetic groups: Tightly/covalently bound cofactors (e.g., haem in peroxidase, biotin in pyruvate carboxylase)
- Coenzymes: Loosely, transiently bound organic cofactors — usually vitamins or their derivatives (e.g., NAD⁺, FAD, CoA, ATP)
- Metal ions: Act as cofactors (e.g., Zn²⁺ in carbonic anhydrase, Fe²⁺/Fe³⁺ in cytochromes, Mg²⁺ in phosphatases)
Apoenzyme: Protein part of enzyme (inactive without cofactor)
Holoenzyme: Apoenzyme + Cofactor = Complete, active enzyme

6.1 Mechanism of Enzyme Action

A. Lock and Key Model (Emil Fischer, 1894)

The active site of an enzyme is rigid and pre-shaped like a lock
Only a specific substrate (the key) fits perfectly into the active site
Explains enzyme specificity but does not explain why closely related substrates can sometimes be acted upon

B. Induced Fit Model (Daniel Koshland, 1958)

The active site is flexible — not rigid
When the substrate approaches, the active site changes shape (induces fit) to accommodate the substrate — like a hand fitting into a glove
More widely accepted — explains broader substrate specificity and allosteric enzymes

📷 Fig. 9.7 (NCERT) Enzyme-substrate complex formation — showing (a) Lock and Key model and (b) Induced Fit model; active site, substrate, ES complex, and product formation NCERT Figure 9.7

6.2 Enzyme Reaction Steps

Step-by-step Mechanism

Substrate (S) binds to active site of enzyme (E) → forms ES complex
Chemical bonds in substrate are broken/formed → EP complex (enzyme-product)
Product (P) is released; enzyme is regenerated unchanged

E + S ⇌ ES → EP → E + P

Turnover number (kcat): Number of substrate molecules converted to product per enzyme molecule per second (Catalytic efficiency)
Enzymes do not alter equilibrium — they only speed up reaching equilibrium from both directions

6.3 Factors Affecting Enzyme Activity

📷 Fig. 9.8 (NCERT) Graphs showing effect of (a) Temperature, (b) pH, (c) Substrate concentration on enzyme activity — showing optimum temperature/pH, Vmax, and Km NCERT Figure 9.8 — combine all three graphs

A. Effect of Temperature

Enzyme activity increases with temperature up to an optimum temperature (usually 37°C for human enzymes; ~60–80°C for thermophilic bacteria)
Beyond optimum: Denaturation — H-bonds and other non-covalent interactions break → active site loses shape → activity falls to zero
Q₁₀ value = activity at (T + 10°C) / activity at T°C ≈ 2 (for most enzymes)

B. Effect of pH

Each enzyme has an optimum pH at which activity is maximum
Pepsin (stomach): optimum pH ~2 (acidic) | Trypsin (intestine): optimum pH ~8 (alkaline) | Most intracellular enzymes: ~7.2–7.4
Extreme pH → ionisation changes in R groups → disruption of H-bonds → denaturation

C. Effect of Substrate Concentration

As substrate concentration increases → enzyme activity increases (more ES complexes formed)
At Vmax (maximum velocity): All active sites are saturated with substrate — further increase in [S] has no effect
Km (Michaelis constant): Substrate concentration at which enzyme velocity = ½ Vmax
Low Km = High affinity of enzyme for substrate | High Km = Low affinity

6.4 Enzyme Inhibition

Type	Description	Effect on Km & Vmax	Example
Competitive Inhibition	Inhibitor resembles substrate; competes for active site; reversible; overcome by high [S]	Km ↑ (apparent); Vmax unchanged	Malonate inhibits succinate dehydrogenase; Sulfa drugs inhibit bacterial DHPS
Non-competitive Inhibition	Inhibitor binds to site other than active site (allosteric site); changes enzyme conformation; cannot be overcome by ↑[S]	Km unchanged; Vmax ↓	Cyanide inhibits cytochrome oxidase; heavy metals (Pb²⁺, Hg²⁺)
Uncompetitive Inhibition	Inhibitor binds only to ES complex (not free enzyme)	Both Km ↓ and Vmax ↓	Rare; seen in multi-substrate reactions
Irreversible Inhibition	Inhibitor permanently inactivates enzyme (covalent bond)	Vmax ↓ (cannot be reversed)	Organophosphates (nerve agents) inhibit acetylcholinesterase; DFP, PCMB
Allosteric Regulation	Regulatory molecules (activators or inhibitors) bind to allosteric site → conformational change	Sigmoid kinetics (not Michaelis-Menten)	ATCase (aspartate transcarbamoylase); Feedback inhibition in metabolic pathways

6.5 Classification of Enzymes (IUB/IUBMB Classification)

Six Classes of Enzymes

Class	Name	Reaction Catalysed	Example
1	Oxidoreductases	Oxidation-reduction reactions; transfer of H or electrons	Dehydrogenases, Oxidases, Reductases, Peroxidases
2	Transferases	Transfer of a functional group from one molecule to another	Kinases (phosphate transfer), Aminotransferases (amino group)
3	Hydrolases	Cleavage of bonds by addition of water (hydrolysis)	Proteases (peptide bond), Lipases (ester bond), Amylases (glycosidic bond)
4	Lyases	Cleavage of bonds by elimination (not hydrolysis); form double bonds	Decarboxylases (CO₂ removal), Aldolases, Dehydratases
5	Isomerases	Interconversion of isomers (intramolecular rearrangement)	Phosphoglucose isomerase, Mutases, Epimerases
6	Ligases (Synthetases)	Joining of two molecules using ATP hydrolysis	DNA Ligase, Aminoacyl-tRNA synthetase, Acetyl-CoA synthetase

⚡ Memory Trick — Enzyme Classes

"Oh, The Happy Lyric Is Lovely"

Competitive inhibitor raises Km (needs more substrate to reach Vmax) but Vmax stays same — "Competitor just slows you down but doesn't stop you"

Non-competitive lowers Vmax — "Non-competitor permanently weakens the team"

🎯 NEET PYQ Focus

NEET 2023

Which type of enzyme inhibition is overcome by increasing substrate concentration?
Answer: Competitive inhibition — inhibitor competes at active site; more substrate displaces it

NEET 2022

Km represents:
Answer: Substrate concentration at which enzyme velocity = ½ Vmax. Low Km = high affinity

NEET 2021

Which of the following is a ribozyme?
Answer: rRNA in ribosome (23S rRNA in peptidyl transferase activity) — RNA with catalytic activity

NEET 2020

What class of enzyme is DNA Ligase?
Answer: Ligase — joins two DNA strands; requires ATP

NEET 2019

Which of the following enzymes has a cofactor?
Answer: Carbonic anhydrase (Zn²⁺ cofactor) — catalyses CO₂ + H₂O ⇌ H₂CO₃

7. School Examination Practice Questions

📝 1 MARK

Q1. What is a coenzyme? Give one example.

Answer: A coenzyme is an organic, non-protein molecule that loosely associates with an enzyme and is essential for its catalytic activity. Coenzymes are usually derived from vitamins.

Example: NAD⁺ (Nicotinamide Adenine Dinucleotide) — derived from Niacin (Vitamin B₃); acts as hydrogen/electron carrier in oxidoreductase reactions.

📝 2 MARKS

Q2. Differentiate between competitive and non-competitive inhibition.

Feature	Competitive Inhibition	Non-competitive Inhibition
Inhibitor binding site	Active site (same as substrate)	Allosteric site (different from substrate site)
Overcame by excess substrate?	Yes	No
Effect on Km	Km increases (apparent)	Km unchanged
Effect on Vmax	Vmax unchanged	Vmax decreases
Example	Malonate vs succinate dehydrogenase	Cyanide vs cytochrome oxidase

📝 3 MARKS

Q3. Describe the four levels of protein structure.

Primary structure: Linear sequence of amino acids joined by peptide bonds (N→C direction). Any change here = different protein (e.g., sickle cell haemoglobin).
Secondary structure: Local folding of polypeptide backbone due to hydrogen bonds → α-helix or β-pleated sheet.
Tertiary structure: Overall 3D shape of a single polypeptide chain; maintained by H-bonds, disulfide bridges, hydrophobic interactions, ionic bonds.
Quaternary structure: Association of two or more polypeptide subunits (protomers) — only in multi-subunit proteins. Example: Haemoglobin (2α + 2β subunits).

📝 5 MARKS

Q4. Explain the effect of temperature, pH, and substrate concentration on enzyme activity. Draw graphs to illustrate.

Temperature: Activity rises with temperature to an optimum (~37°C in humans); beyond this, enzyme denatures and activity drops. Thermophiles have optima at 60–80°C.

pH: Each enzyme has an optimum pH. Pepsin works at pH 2; Trypsin at pH 8; salivary amylase at pH 6.8. Extreme pH causes denaturation.

Substrate concentration: Activity increases with [S] until Vmax is reached (all active sites saturated). The Km is the [S] at ½Vmax — measure of enzyme-substrate affinity.

8. Quick Revision — One-Liners

⚡ Rapid Fire Revision

Most abundant organic compound on Earth → Cellulose
Most abundant protein in animals → Collagen
Most abundant protein in the world → RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase)
Storage carbohydrate in plants → Starch | in animals → Glycogen
Reducing sugars → Glucose, Fructose, Maltose, Lactose | Non-reducing → Sucrose
Peptide bond → Covalent bond; between —COOH and —NH₂ with loss of H₂O
Number of standard amino acids → 20
Smallest amino acid → Glycine (R group = H; only achiral/optically inactive amino acid)
Amino acid with sulphur in R group → Cysteine and Methionine
Disulfide bond → between two Cysteine residues (S–S bond) — tertiary/quaternary structure
Secondary structure stabilised by → Hydrogen bonds
Protein with all 4 levels of structure → Haemoglobin (quaternary)
DNA double helix — pitch → 3.4 nm; base pairs/turn → 10; diameter → 2 nm
A=T → 2 H-bonds | G≡C → 3 H-bonds
Purines → Adenine, Guanine (PURE As Gold) | Pyrimidines → Cytosine, Thymine, Uracil (CUT — C, U, T)
Nucleoside = Sugar + Base | Nucleotide = Sugar + Base + Phosphate
Ribozyme → Catalytic RNA; e.g., 23S rRNA, Ribonuclease P, Hammerhead ribozyme
Lock and Key → Fischer (1894) | Induced Fit → Koshland (1958)
Holoenzyme = Apoenzyme + Cofactor
Competitive inhibitor → Km ↑, Vmax same | Non-competitive → Km same, Vmax ↓
Optimum pH of Pepsin → pH 2; Trypsin → pH 8; Salivary amylase → pH 6.8
Q₁₀ of enzyme reactions ≈ 2 (activity doubles with every 10°C rise)
6 classes of enzymes: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases
Chitin → β-1,4-glycosidic bond between NAG units (like cellulose)
Chargaff's rule → A + G = T + C (total purines = total pyrimidines) in DNA
Watson and Crick proposed DNA structure → 1953
Essential fatty acids → Linoleic acid (ω-6) and Linolenic acid (ω-3)
Phospholipid → amphipathic molecule; forms bilayer of biological membranes
Steroid hormones are derived from → Cholesterol

🧠 Master Memory Tricks

Purines vs Pyrimidines: "PURE As Gold" → PURines = Adenine + Guanine (2 rings) | "CUT the PY" → PYrimidines = Cytosine, Uracil, Thymine (1 ring)

DNA vs RNA bases: DNA has Thymine = DNA Tyrant; RNA has Uracil = RNA Uses Uracil

Enzyme classification order: "Oh, The Happy Lyric Is Lovely" → Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases

Protein secondary structure: "α-helix is like a SPRING (3.6 aa/turn)" | "β-sheet is like a PLEATED SHEET (flat)"

For Km: "Km is INVERSELY related to AFFINITY" — Low Km = High affinity = enzyme doesn't need much substrate

Sickle cell haemoglobin: Position 6 of β-chain — Glutamate (Glu, polar) → Valine (Val, nonpolar) — point mutation in primary structure causes complete disease

🔑 NEET High-Yield Summary

Most Tested Topics from Biomolecules in NEET

1. Enzyme inhibition — Competitive vs Non-competitive (Km, Vmax changes)
2. DNA structure — base pairing, H-bonds, Chargaff's rule, Watson-Crick model parameters
3. Protein structure — 4 levels, bonds involved, examples
4. Polysaccharides — Starch (amylose vs amylopectin), Cellulose, Glycogen, Chitin, Inulin
5. Enzyme classification & cofactors — Know all 6 classes and examples of coenzymes

Class 11 Biology | Chapter 9

BIOMOLECULES

1. Chemical Analysis of Cells

1.1 Primary vs Secondary Metabolites

2. Carbohydrates

2.1 Classification of Carbohydrates

A. Monosaccharides (Simple Sugars)

B. Oligosaccharides

C. Polysaccharides (Biomacromolecules)

3. Proteins

3.1 Amino Acids — Building Blocks of Proteins

3.2 Peptide Bond

3.3 Four Levels of Protein Structure

3.4 Classification of Proteins

4. Lipids

4.1 Classification of Lipids

A. Fatty Acids

B. Triglycerides (Fats and Oils)

C. Phospholipids

D. Steroids (Sterols)

E. Waxes

5. Nucleic Acids

5.1 Nucleotide Structure

5.2 DNA vs RNA

5.3 Structure of DNA — Watson and Crick Model (1953)

6. Enzymes — Detailed Study

6.1 Mechanism of Enzyme Action

A. Lock and Key Model (Emil Fischer, 1894)

B. Induced Fit Model (Daniel Koshland, 1958)

6.2 Enzyme Reaction Steps

6.3 Factors Affecting Enzyme Activity

A. Effect of Temperature

B. Effect of pH

C. Effect of Substrate Concentration

6.4 Enzyme Inhibition

6.5 Classification of Enzymes (IUB/IUBMB Classification)

7. School Examination Practice Questions

8. Quick Revision — One-Liners